This invention relates to pulsed droplet deposition apparatus and more particularly to such apparatus including a plurality of droplet deposition channels. Typical of this kind of apparatus are multi-channel pulsed droplet ink jet printers, often also referred to as "drop-on-demand" ink jet printers.
An existing technology for the production of multi-channel drop-on-demand ink jet printers is known from, for example, U.S.-A-3,179,042; GB-A-2 007 162 and GB-A-2 106 039. These patent specifications disclose thermally operated printheads which, in response to an electrical input signal, generate a heat pulse in selected ink channels to develop a vapour bubble in the ink of those selected channels. This in turn generates a pressure pulse having the pressure and time characteristics appropriate for the ejection of an ink droplet through a nozzle at the end of the channel.
Thermally operated printheads of this nature possess a number of significant disadvantages. First, the thermal mode of operation is inefficient and typically requires 10 to 100 times the energy to produce an ink droplet as compared with known piezo-electric printheads. Second, difficulties are found in providing the very high levels of reliability and extended lifetimes which are necessary in an ink jet printhead. For example, thermally operated printheads have a tendency for ink deposits to form on the heating electrodes. Such deposits have an insulating effect sufficient to increase substantially the electrical pulse magnitude necessary to eject an ink droplet. Thermal stress cracks and element burn-out, as well as cavitation erosion, have also proved difficult to eliminate. Third, only ink specifically developed to tolerate thermal cycling can be used and suitable ink formulations often proved to be of low optical density compared with conventional inks.
Attempts have been made to produce multi-channel ink jet printers using piezo-electric actuators and reference is made in this connection to U.S.-A-4,525,728; U.S.-A-4,549,191 and U.S.-A-4,584,590 and IBM Technical Disclosure Bulletin Vol. 23 Mar. 10, 1981. Piezo-electric actuators have the advantage, compared with thermal processes, of low energy requirement. However, the existing proposals have not achieved the levels of printing resolution that are desired. A prime influence upon printing resolution is the number of channels, and thus nozzles, per unit length in the direction transverse to paper movement relative to the head. Existing piezo-electric printhead technology as exemplified by the prior art referenced above, is capable of achieving a maximum channel density of around 1 to 2 channels per mm. In terms of effective resolution, and by this is meant the density at which the droplets can be deposited upon paper, such nozzle density is for many applications insufficient. It does not, for example, enable a transverse line to be printed with ink droplets that are indistinguishable by the eye at normal reading distance.
Effective resolution can be increased, for example, by angling the printhead in the plane of the paper so as to decrease the inter-channel spacing in the transverse direction. However, this necessitates sophisticated control logic and the use of delay circuitry to ensure that all droplets associated with a particular print line are deposited on the paper in a single transverse line (or sufficiently close to the line to be indistinguishable therefrom by the eye). An alternative approach is to provide for movement of the printhead. As will be understood, this introduces significant mechanical and control complexities, and is not felt to be advantageous. A third approach to increasing effective resolution is to provide two or more banks of channels which are mutually spaced in the direction of paper movement but which cooperate to print a single transverse line. With only two such banks it may be possible to configure the nozzles of both channels in a common print line. With more banks, a significant nozzle spacing is built up in the direction of paper movement and delay circuitry is required to provide for the time spaced actuation of the channels necessary for spatial coincidence. The provision of delay circuitry adds to manufacturing costs by an amount which typically increases with the amount of delay required.
It is useful to note at this point that colour printing would typically require four banks of channels even if each bank provided in itself sufficient single colour resolution. Where a multiplicity of banks are required to produce the desired resolution for a single colour, it will be understood that colour applications compound the problems outlined above.
The advantages of decreasing the inter-channel spacing in the direction transverse to relative paper movement should now be apparent. In many cases, typically where colour printing is required, there are further advantages in reducing the inter-channel spacing along the direction of paper movement (that is to say between banks). This reduces the bulk dimensions of the printhead but more importantly reduces the time delays necessary for spatial coincidence.